1. Technical Field
The present invention relates to a system for controlling a multipoint conference, and more particularly to a video conference control system using an integrated services digital network (ISDN) for providing a video conference involving at least three or more video terminals.
2. Background Art
A video conference system is an advanced multimedia system in which a conversation or video conference is made among distant locations in order to quickly transmit information and to reduce both time and cost of business trip. Traditionally, a video conference is established between two terminals on a point-to-point basis. Each terminal, that is, each conference room, includes a microphone for generating an audio signal for transmission to the other terminal, a receiver for receiving an audio signal form the other terminal, a video camera for generating a video signal for transmission to the other terminal and a display device for display a video signal generated at the other terminal. Each terminal also includes a conference control unit which controls the coding of the video signal and the audio signal for transmission in a compressed fashion to the other terminal and for decoding a coded video signal and audio signal received from the other terminal. The conference control unit typically realizes an interactive conference between two terminals.
As advances in digital compression and availability of international standards and digital networks are made in recent years, there is a demand to realize a multi-point video conference system which enables a conference among three or more terminals. Typically, the conventional multi-point video conference system as disclosed, for example, in U.S. Pat. No. 5,003,532 for Multi-Point Conference System issued to Ashida et al., U.S. Pat. No. 5,315,633 for Digital Video Switch For Video Teleconferencing issued to Champa, and U.S. Pat. No. 5,325,194 for Multipoint Video Conferencing System issued to Natori et al, is implemented using a centralized multipoint control unit (MCU) connected through a star network to conference room terminals provided in each terminal. The multipoint control unit (MCU) carries out selection and distribution of images, mixing of voices, and centralized control of a whole system.
The MCU automatically selects a conference room, for example, where a speaker is present, as a master terminal by detecting a speech signal, and distributes an image of the master terminal to other participating terminals. To the master terminal, an image of a secondary participating terminal is distributed. The MCU then automatically sets a new master terminal in response to a request from an arbitrary terminal prior to receiving the request as a new secondary terminal and sets all terminals other than the new master and secondary terminals as new participating terminals. Such a conventional multi-point video conference system, however, requires a complex internal bus structure which restricts the transmission speed of the communication. Moreover, it has been my observation that T1-class leased line is necessarily used as a line interface in the conventional multipoint video conference system for interfacing participating terminals. This T1-class leased line interface, however, requires costly installation. Further, the conventional multipoint video conference system does not conform to international standards which make it less compatible with other communication terminals.
Recent improvement over the conventional multipoint video conference system in which the use of MCU is avoided is disclosed, for example, in U.S. Pat. No. 5,365,265 for Multipoint Teleconference System Employing Communication Channels Set In Ring Configuration issued to Shibata et al. In Shibata '265, an integrated services digital network (ISDN) as stipulated in the International Telegraph and Telephone Consultative Committee Recommendation (CCITT) I-Series is used as line interface in order to reduce cost while concomitantly improving the transmission speed of the communication in the system. While the multipoint video conference system of Shibata '265 has merits in its own right, it is my opinion that further improvements can be contemplated to reduce installation cost of line interface and to simplify the internal data bus structure.